The invention relates to heat exchange tubing and particularly to heat exchange tubing for use in solar applications where, for example, a can or shell member might have drinking quality water flowing through it to be heated by an internal heat exchange coil through which an ethylene glycol solution, for example, is circulated. The coil is typically in series with the flow channels of a solar collector element. In order to insure that the liquid in the can and the liquid in the coil do not mix in the event of a tube wall failure it has been proposed that the coil comprise a double wall tube. Although a double walled tube having its two walls in complete, intimate contact throughout their length would seem to provide good heat transfer, although not as good as a single wall tube, such a design would not enable one to tell when an opening in either wall developed. Thus, it is necessary to provide a space between the walls through which leakage through an opening in either wall can flow. However, the existence of such a space could be expected to reduce the heat transfer efficiency of the coil. Thamasett et al. U.S. Pat. No. 3,830,290 illustrates the use of pyramidshaped spacers on at least one of a pair of concentric pipes and a leakage indicator means sensitive to an increase in pressure in the leakage space between the pair of pipes. The pipes are plain and have no surface enhancement features. Kuthe U.S. Pat. No. 2,913,009 and Nakayama Canadian Pat. No. 736,374 each show composite tube assemblies with the outer tube having external fins and the inner tube having an enhanced inner surface to increase turbulence. In each case, the tube assemblies are designed to enhance internal and external heat transfer and there is no suggestion of a flow channel for leakage between the tubes. Thus, one would never know it if one or the other tube developed an opening through its wall. In Kuthe, the tubes have contacting plain sections at their ends which would prevent any flow from between the tubes and thus would prevent their use as leak detectors. In Nakayama, there is no space between the tubes in the FIG. 3 embodiment and the patentee suggests that the groove 6 could be filled with heat conducting material, thus preventing leakage detection.